1. Field of the Invention
The present invention relates generally to thin film processes, more specifically to thin film processes for the fabrication of ink-jet printhead structures, and particularly an improved method for fabrication of thermal ink-jet printhead drop generator arrays and an ink-jet printhead fabricated in accordance with the method.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
A simplistic schematic of a swath-scanning ink-jet pen 100 is shown in FIG. 1 (PRIOR ART). The body of the pen 101 generally contains an ink accumulator and regulator mechanism 102. The internal ink accumulator—or ink accumulation chamber—and associated regulator 102 are fluidically coupled 103 to an off-axis ink reservoir (not shown) in a known manner common to the state of the art. The printhead 104 element includes appropriate electrical connectors 105 (such as a tape automated bonding, “flex tape”) for transmitting signals to and from the printhead. Columns of individual nozzles 106 form an addressable firing array 107. The typical state of the art scanning pen printhead may have two or more columns with more than one-hundred nozzles per column. The nozzle array 107 is usually subdivided into discrete subsets, known as “primitives,” which are dedicated to firing droplets of specific colorants on demand. In a thermal ink-jet pen, an ink drop generator mechanism includes a heater resistor subjacent each nozzle 106 with an ink chamber therebetween. Selectively passing current through a resistor superheats ink to a cavitation point such that an ink bubble's expansion and collapse ejects a droplet from the associated nozzle 106.
Prior art for printhead structures and fabrication is typified by patents to Keefe et al., assigned to the common assignee herein. U.S. Pat. No. 5,278,584 shows an IMPROVED INK DELIVERY SYSTEM FOR AN INK-JET PRINTHEAD. U.S. Pat. No. 5,635,966, a continuation in part of the Keefe '584 patent, shows an EDGE FEED INK DELIVERY THERMAL INKJET PRINTHEAD STRUCTURE AND METHOD OF FABRICATION.
The ever increasing complexity and miniaturization of TIJ nozzle arrays has led to the use of silicon wafer integrated circuit technology for the fabrication of printhead structures. For the purpose of the present invention, the “frontside” of a silicon wafer, or wafer printhead die region, is that side having drop generator elements; the “backside” of a silicon wafer, or wafer printhead die region, is the opposite planar side, having ink feed channels (also referred to simply as “trenches”) fluidically coupled by ink feed holes through the silicon wafer to the drop generator elements. It is generally desirable in any integrated circuit (IC) thin film process to minimize masking steps to reduce cost and complexity.
FIG. 2 (PRIOR ART) is an illustration of a highly magnified cross-section of a thermal ink-jet printhead structure 200. It should be recognized that these illustrations are schematics for a very small region of a silicon wafer which may be many orders of magnitude greater in dimension to the shown die region. Many publications describe the details of common techniques used in the fabrication of complex, three-dimensional, silicon wafer based structures; see e.g., Silicon Processes, Vol. 1–3, copyright 1995, Lattice Press, Lattice Semiconductor Corporation (assignee herein), Hillsboro, Oreg. Moreover, the individual steps of such a process can be performed using commercially available fabrication machines. The use of such machines and common fabrication step techniques will be referred to hereinafter as simply: “in a known manner.” As specifically helpful to an understanding of the present invention, approximate technical data are disclosed herein based upon current technology; future developments in this art may call for appropriate adjustments as would be apparent to one skilled in the art.
Historically, the thin film process for forming such a structure 200 consisted of a nine mask process, four for transistor(s) formation and five for ink drop generator(s) formation. In order for the transistor formation are the active region mask, the polysilicon mask, the contact mask, and the substrate contact mask. The “substrate contact” is used to ground the silicon and the body of the MOSFET devices.
An orifice plate 201 overlays a printhead barrier layer 203 in a manner such that ink 205 from a supply (not shown) accumulates in a drop firing chamber in a nozzle 106 (FIG. 1) superjacent a heater/firing resistor 207. An electrical contact lead 209, in this embodiment a layer of gold 209′ superjacent a layer of tantalum 209″, is connected via an aluminum/tantalum-aluminum trace 211 to a MOSFET 213 device formed in the surface of a silicon substrate 215. The MOSFET device 213 is coupled to the firing resistor 207 via another aluminum/tantalum-aluminum trace 211′. Control signals to the transistor 213 selectively turn such heater resistors on and off to eject ink drops from the array 107 (FIG. 1) in accordance with the digital date for dot matrix printing.
In forming the heater/firing resistor driver MOSFET 213 as shown in FIG. 2 the contacts and substrate contacts in the state of the art are formed by the steps shown in FIGS. 3A, 3B and 3C (PRIOR ART). FIG. 3A shows a cross-section depiction having a plurality of partial formed MOSFETS immediately after the contact etch step has been performed. Based on a superjacent photoresist mask layout of a third mask in the overall process, this contact etch step selectively removes phosphosilicate glass (“PSG”) down into the source/drain down to the source/drain regions of the doped substrate so that in subsequent steps, when aluminum/tantalum-aluminum for the traces 211, 211′, FIG. 2, is deposited, the metal is in contact with each source/drain region. The contact etch also makes a hole in the PSG over the substrate contacts, but the etch stops on the polysilicon 301. As demonstrated by FIGS. 3B and 3C, a separate photoresist mask 303 (fourth, or “substrate contact”) must be used to etch the polysilicon and gate oxide to create a substrate contact, metal-to-silicon. In other words, note that substrate contacts require a special mask because the contacts have to go through an oxide, PSG, polysilicon, and gate oxide. Thus, it is important to note that the contact etch cannot be used by itself to make the substrate contacts because if the etch reaction is changed to also remove the polysilicon superjacent the substrate contact region, it would etch into the silicon in the source/drain contacts. At best, this would at least create unacceptable reliability problems during operation. At the worst it could make the device unusable, destroying wafer yield.
Thus, there is a need for an improved process for fabricating thermal ink-jet printheads.